1. Field of the Invention
The invention in general relates to steam turbine control systems and more particularly to overspeed protection control circuitry for deriving extremely accurate turbine speed signals.
2. Description of the Prior Art
In the field of steam turbine control, many systems exist which utilize a primary controller, in the form of a programmable digital computer, as well as a redundant or backup computer. The computer's capability to monitor, memorize, calculate, test and make instant decisions results in a control system which is faster, more accurate and far superior to purely mechanical or analog control systems.
An improved digital control system for a steam turbine has been developed which includes primary and redundant base controllers as well as interconnected and coordinated functional modules each having its own microcomputer to execute specific functions. That is, the control system structure is based upon distributed processing, with this modular architecture providing for greater flexibility and minimizing risk of control loss and total system shutdown due to any single failure. The system can be serviced while on-line without the necessity for shutting down the turbine's operation, and servicing of the apparatus can be accomplished in a minimal amount of time. One example of such distributed processing turbine control system is described and claimed in U.S. Pat. No. 4,368,520 assigned to the assignee of the present invention and hereby incorporated by reference.
The control system of the referenced patent includes a plurality of valve position control circuits for controlling the steam admission valves, with each circuit including its own programmable digital computer in two-way digital communication with a base controller from which it receives signals relative to the individual valve control. The valve position control circuits are selectively addressable to receive a particular valve related signal from the controller to in turn generate an individual valve drive signal for the valve it is controlling. The system is operable both in an automatic and a manual mode and when in the manual mode all of the valve position control circuits function to receive operator-entered command signals.
Two channels of overspeed protection control (OPC) are provided with each OPC including its own programmable digital computer and operable in response to certain speed indicative pulse input signals to generate presumed valid RAM turbine speed signals which are used for control purposes and are communicated via a digital data link to the base controller.
The present invention relates to an improved OPC arrangement which provides for a faster response time and higher precision in the speed measurement, resulting in a precise speed measurement signal with a resolution of better than 1 RPM. In addition, the improved OPC arrangement is operable with different rated speeds which may be encountered in different turbine systems such as boiler feed pump turbines, fossil-fired, and nuclear, by way of example.